Aromatic polycarbonates based on bis-(hydroxyphenyl)-alkanes, especially 2,2-bis-(4-hydroxyphenyl)-propane, can be converted into films which are used, in particular, as electrical insulating films in the electrical industry (see, for example, DT-PS 971,790; Angew. Chem. 74, 1962, pages 647 to 650 ).
As a rule, such films are manufactured from solutions by casting. They can, however, also be obtained by extrusion through slit dies. If such films come into contact with organic non-solvents, for example carbon tetrachloride, their mechanical properties can be severely impaired, especially if the films are exposed to stresses during the action of such non-solvents. This can reach the point of the films breaking after a relatively short time. This property is known to experts as "stress-cracking."
Since, when using the electrical insulating films, the insulated parts may come into contact with non-solvents, for example, for the purpose of removing traces of grease and oil, it is only possible to use electrical insulating films whereof the proneness to stress-cracking is as low as possible.
As the viscosity of polycarbonates increases, the films manufactured therefrom show a reduction in the proneness to stress-cracking. At a relative viscosity of 1.70 (0.5 g in 100 ml of methylene chloride solution), corresponding to an average molecular weight of about 70,000, the proneness to stress-cracking is so slight that the usability of the films is no longer impaired.
Polycarbonates having such a high viscosity, however, cannot be processed into films on extuders, since at the high processing temperatures required for the purpose the polycarbonate is so severely damaged with an associated loss of an important part of the properties, that the material can no longer be used as an electrical insulating film. Hence, only polycarbonate of substantially lower viscosities can be extruded to give films. All extruded polycarbonate films corresponding to the state of the art, therefore, are unable to be used as an electrical insulating film, or can only be used to a very restricted degree, because of their proneness to stress-cracking
Polycarbonate films which are used for the insulation of electrical coils in addition to having a low susceptability to stress-cracking, must also be resistant to styrene solutions of unsaturated polyester casting resins since these coils are frequently potted in such casting resins. The known extruded polycarbonate films, because of their low viscosity, do not show sufficient resistance towards the styrene solutions of the unsaturated polyester casting resins used. They can, therefore, not be used for coil insulation.
Cast polycarbonate films and extruded polycarbonate films tend to adhere strongly to themselves. This phenomenon is known to experts as the "blocking effect" or "stick-slip effect."
This effect interferes with the manufacture and especially the processing of the films. This adhesion is circumvented, for example, by roughening one side of the films during manufacture. It is also known that the mutual adhesion of films can largely be prevented by additives, for example of silicone dioxide or talc.